(1) Field of the Invention
The present invention relates to a image forming apparatus that forms an image by transferring a toner image from an image holding belt such as an intermediate transfer belt, to a transfer sheet.
(2) Description of the Related Art
Color image forming apparatuses that are capable of copying and/or printing full color images using electronic photographing methods have come into practical use in recent years. One technique used to transfer an image to a transfer sheet in a color image forming apparatus is an intermediate transfer method. With an intermediate transfer method, yellow (Y), magenta (M), cyan (C) and black (B) images that have been formed separately on a photosensitive body are superimposed on each other in turn on an intermediate transfer belt in accordance with reference marks on the intermediate transfer belt, and then the entire resultant full color toner image is transferred from the intermediate transfer belt to a transfer sheet by a transfer roller (secondary transfer).
Since transfer to the transfer sheet only needs to be performed once, an image forming apparatus that uses this kind of intermediate transfer method has the advantage of obtaining stable image quality and an ability to deal with various types of paper compared to an image forming apparatus that must perform a plurality of transfers to a transfer sheet directly.
However, in an image forming apparatus that uses an intermediate transfer method, if the transfer sheet and the intermediate transfer belt do not contact each other closely at a place just before the transfer sheet enters a transfer nip formed between the transfer roller and the transfer belt, and hence a small gap exists therebetween, the transfer sheet may touch the transfer roller directly at the place before the transfer nip. This will cause the toner to spatter due to the electric field generated between the transfer sheet and the intermediate transfer belt, resulting in problems such as poor image quality, and also cause discharge to occur in parts, resulting in pin holes in the image.
Various attempts have been made to avoid such problems, one of which is the technique disclosed in Japanese Unexamined Patent Application Publication No. H05-197241. This technique stipulates a range of 0° to 40° for an angle between the surface of the intermediate transfer belt and a guide surface of a pre-transfer guide that guides the transfer sheet to the transfer nip, ensuring that the transfer sheet is more likely to be in close contact with the intermediate transfer belt before the transfer nip.
However, the inventors of the present invention discovered that simply ensuring that the angle between the guide surface of the pre-transfer guide and the intermediate transfer belt is in a range of 0° to 40° is not sufficient to maintain contact between the intermediate transfer belt and the transfer sheet stably before the transfer nip, or to prevent poorness of transfer.
Such poorness transfer occurs when transferring the toner image from the substantially belt-shaped image holding body (image holding belt) to the transfer sheet, and therefore are not unique to color image forming apparatuses, but also occur in monochrome image forming apparatuses that use an image holding belt. These problems can also occur when the image holding belt is a photosensitive belt, and a toner image is formed by direct exposure scanning of the belt and transferred to a transfer sheet.